International Electrical Engineering Journal (IEEJ)

International Electrical Engineering Journal (IEEJ)

Vol. 6 (2015) No.X, pp. XX-XX

ISSN 2078-2365



Performance and Analysis of Direct Torque Control Induction Motor FED IEEJ Published Paper

Author 1, Author2, Author3

Affiliation University

Author @gmail.com

Abstract—AC asynchronous electrical motors especially squirrel cage induction motor presents the greater advantages on cost and energy efficiency as compared with other industrial solutions for varying speed applications.The force-commutated power electronics switches (GTOs, MOSFETs, and IGBTs etc) based inverters are widely employed these days in the conversion of constant speed into variable speed induction motors. Due to the invention of these advanced power electronics switches (GTOs, MOSFETs, IGBTs etc) with improved characteristics, the PWM inverter fed induction motors are widely replacing DC motors and thyrister bridges day by day in the industries. The torque control or field control method may be used with PWM inverter fed induction motors jointly and can achieve the same flexibility in speed and torque control as with DC motors. In this present paper, an IGBT inverter fed squirrel cage induction motor with torque control technique is developed and simulated in the recent MATLAB/Simulink environment and observed the transient characteristics of the used motor. Many motor parameters have been used for induction motor analysis purpose. Further, V/f control and Total Harmonics Distortion (THD) analysis has also been carried out. Though, the stator current parameter of the motor has been widely used from last three decades with Motor Current Signature Analysis (MCSA) technique. Therefore, in the THD analysis, only stator current motor parameter has been analysed.

Index Terms— Squirrel cage induction motor (SCIM), Induction Motor (IM), Pulse width modulation (PWM), Insulated Gate Bipolar Transistor (IGBT), Time Domain Analysis, Direct Torque Control (DTC), V/f Control, MATLAB/Simulink

I.Introduction

Earlier, the DC motors were widely employed in industries to achieve variable speed for many applications by changing the field and armature current. Consequently, its flux and torque controlled efficiently. But this method requires frequent maintenance, high cost, unable to use in the explosive environment. By using DC motors, large variation in the motor’s speed is not possible. Therefore, for wide range of speed control purpose induction motor fed PWM inverter is preferred these days. Nowadays, induction motor has become the heart of industries with modern power electronics devices. By using this combination, IM can control the wide range of speed with very competitive pricing. Therefore, the induction motor has become the “workhorse” of the motion industry [1-4,13].

In the past, the induction motors were widely employed in constant speed applications because of the unavailability of the variable- frequency voltage supply. The variable- frequency voltage can achieveby theadvanced power electronics semiconductor devices based inverters. These inverters widely called Pulse Width Modulation (PWM) Inverters. These inverters provide us capability to vary the frequency of the voltage supplies in the relatively easy way. As a result, the induction motor can be used in the variable speed drive applications. The PWM inverter may control the large amount of speed of the induction motor[1,4,7,13].

The PWM inverters covered many applications in the electrical engineering field such as UPS, AC electric drives, HVDC reactive power compensators in power systems and communications to power control and conversion. In the present time, the PWM inverter fed asynchronous motor drives are widely used in the industriesbecause it provides more variable and offer in a wide range of speed control with better efficiency and higher performance compared to constant frequencymotor drives[4-6,14].

The induction motor with advanced power electronics inverters not only controls the motor speed but can also pick up motor’s dynamic as well as steady state characteristics. Subsequently, these devices can reduce the system's average power consumption and noise generation of the induction motor [1,7].

The three phase Squirrel Cage Induction Motor (SCIM) is simple, efficient and robust asynchronous motor and often a natural choice as a drive for industries with a very competitive pricing. Squirrel Cage Induction motors (SCIM) are the most extensively used electric motors for appliances, industrial control, automation and transportation. The SCIM widely employed these days as variable speed induction motor with PWM inverters and replacing DC motors and thyristor bridges in industries [1, 3, 5].

It has been seen that many researchers proposed various IM models with different power electronics devices based inverters and control the speed of the IM. But harmonics always creates problems in the currents, voltage and developed electromagnetic torque. Therefore, the modeling of such motors is still being the challenging task [1, 8-12,14].

The IM with PWM inverters has been delivering challenging competition with D.C motors these days and given the cause for replacement of D.C. motors from very fast speed in the industries. This technique is cheap and safe to achieve variable speed efficiently with less power consumption and minimum maintenance. Consequently save many dollars for industries. The IM cost per KVA is approximately one fifty than DC motors. Therefore, it seizes higher appropriateness in antagonistic atmosphere [1-3].

In the present paper, we have proposed and developed an SCIM fed PWM inverter model in the recent MATLAB/Simulink environment. We have used IGBT inverter and torque control method with induction motor jointly for better results with reduced harmonics. The V/f control has also been carried out for the proposed model. The transient behavior of the induction motor has been deeply observed. In future, this model may be used in the various power electronics and drive applications.

II. MATHEMATICAL MODELLING of INDUCTION MOTOR

The traditional per-phase equivalent circuit is used for induction motor mathematical modelling purpose. The basic diagram of per phase equivalent circuit forq and d-axis is as shown in Fig. 1. The equations are derived in the Clarke or αβ (stationary) reference frameby d and q variables. The d-q transformation is widely used because it reduces three AC quantities into two imaginary DC quantities for balanced circuits. The simplified calculations maybe carried out on these two imaginary DC quantities for recoveringthe actual three-phase AC results by performing inverse transformation.

(a)

(b)

Fig. 1 Per- Phase Equivalent Diagram of IM, (a) q-axis, (b) d-axis

The stator circuit voltage equations in theClarke or αβ (stationary) reference frametransformationis as follows:

(1)

Where,

(2)

The stator voltage equations involved in the conversion of rotating reference into stationary reference frame are as follows:

(3)

The variable stationary reference and variable rotating reference frame expressed by the following equations (4 & 5):

(4)

Where,

(5)

The q-axis and d-axis stator voltage equations are as follows:

(6)

(7)

Theq-axis and d-axis rotor voltage equations are as follows:

(8)

Where,

(9)

For simulation of an open loop model, following flux linkage equations (10 to 13) would be considered.

(10)

(11)

(12)

(13)

In our case, for closed loop model following flux linkage equations (14 to 16) will be considered.

(14)

(15)

(16) The total stator and rotor inductances may be expressed by the following equations:

(17)

(18)

The developed electromagnetic torque may be written by the following equation:

(19)

The rotor acceleration equation may be expressed by the following equations:

(20)

Where,, and

The abc to dq reference frame transformations in the phase-to-phase voltage may be expressed by the following equations. The q-axis transformed stator voltage equation is as follows:

(21)

Where,,

The d-axis transformed stator voltage equation is as follows:

(22)

Where,,

The q-axis transformed rotor voltage equation is as follows:

(23)

Where,,

The d-axis transformed rotor voltage equation is as follows:

(24)

Where,,

In the preceding equations, θ is the angular position of the reference frame, while δ= θ - θr is the difference between the position of the reference frame and the position (electrical) of the rotor. Because the machine windings are connected in a three-wire Y configuration, there is no homopolar (0) component. This also justifies the fact that two line-to-line input voltages are used inside the model instead of three line-to-neutral voltages.

The dq to abc reference frametransformations applied for the phase currents may be expressed by following equations:

The phase a current in the stator side may be expressed by the following equation:

(25)

Where,,

The phase b current in the stator side may be expressed by the following equation:

(26)

Where,

,

The phase a current in the rotor side may be expressed by the following equation:

(27)

Where,,

The phase b current in the rotor side may be expressed by the following equation:

(28)

Where,

,

The phase c current in the stator side may be expressed by the following equation:

(29)

The phase c current in the rotor side may be expressed by the following equation:

(30)

The complete mathematical modelling of the used induction motor may be understood by equations (1 to 30).

III.Proposed Direct Torque Control Induction Motor Fed PWM Inverter

The force-commutated electronic switches are widely employed these days to convert constant speed asynchronous motors into variable speed asynchronous motors. Power electronics switches such as MOSFETs, GTOs and IGBTs efficiently controls the induction motor. The Pulse Width Modulation (PWM) voltage source(VSC) inverter-fed induction motor are widely used these days and gradually replacing DC motors and thyristor bridges. With PWM inverter, combined with advanced control strategies such as field-oriented control or direct torque control, we can achieve the same flexibility in speed and torque control as with DC machines. Since, it has been observed that the IGBTs inverter fed into induction motor give better results rather than other power electronics switches based inverters fed induction motor in many power applications with reduced harmonics. Therefore, in the present work, the torque control simulation model has been developed in the MATLAB with PWM inverter fed induction motor. The Insulated Gate Bipolar Transistor (IGBT) inverter has been used in the conversion of DC voltage into AC voltage.

The proposed inverter fed induction motor simulation model is as shown in Fig. 2. The positive load torque ought to be applied on the shaft, if electric motor operates in the motoring mode or negative for generating mode. The nominal full load positive torque is applied on the machine’s shaft is 15 Nm for motoring mode. The SCIM block available in the MATLAB/Simulink having three inputs and one output. The three phase supply would be given in the three inputs and output side one bus selector has been used. The bus selector de-multiplexes many outputs of induction motor. There are 21 motor signatures are hidden in induction motor’s one output. We can de-multiplex these signals as per our requirement. In our case, we have been de-multiplexed only maximum six parameters for analysis purpose.

Fig. 2 Proposed DTC Control SCIM Fed PWM Inverter Simulation Model

In the present paper, a three-phase SCIM rated 3 HP, 220 V, 1430 RPM is fed by a sinusoidal PWM inverter has been considered for analysis purpose. The sinusoidal reference wave base frequency is used 50 Hz while the triangular carrier wave's frequency is calculated and set to 1650 Hz. The modulation factor is 33 and ought to be choose as high as possible for better results. Therefore, the triangular carrier wave's frequency corresponding to the given modulation factor is 50×33=1650 Hz. Since, the switching frequency (1650 Hz) is relatively high. Therefore, maximum time step is restricted to 10 µs. This high switching frequency (1650 Hz) is required for the inverter. It is recommended in [2-3] that the modulation factor (mf) ought to be an odd multiple of three and that value has to be as high as possible.

The machine's rotor is short-circuited for SCIM. The smoothing reactor has also been placed between the inverter and the electric machine for simulating the effect of a smoothing reactor. Consequently, we have set the stator leakage inductance twice to its actual value. If we use PWM inverter then noise will be introduced in the electromagnetic torque,voltage and currents but the motor's inertia will prevents the noise from appearing in the motor's speed.

The Clarke or αβ transformation (stationary reference frame) is used in this work. The reference frame is required for the conversion of input voltages (abc reference frame) into the output currents (dq reference frame). If we are interested to do park transformation then the rotor reference frame ought to be choose.

In the stationary reference frame, the rotor angle value is set to 0 and the value of δ is set to -θr. The reference frame affects the simulation speed, waveforms of all dq variables and in some certain cases the accuracy of the obtained results.

The brief descriptions of the used SCIM block parameters are given in appendix (Table I).

The snubber circuit is essential with IGBT and provided inbuilt in the MATLAB/Simulink Library. In the used IGBT block, we have set the value of snubber capacitor infinite (short- circuit). It means the purely resistive snubber has been used. Generally, the Insulated Gate Bipolar JunctionTransistor (IGBT) do not use snubbers but every non-linear elements are used in the MATLAB/Simulink is modeled as a current source. Consequently, we need to provide a parallel path across each IGBT to allow connection to an inductive circuit (stator of an induction motor). But, the circuit performance would not be affected by choosing high value of the snubber. A pulse generator is employed to control the inverter bridge.

In the present work, the torque control method has been used for induction motor controlling purpose with PWM inverter. Since, the field or vector control method is a pretty attractive control method but due to some serious drawbacks is not used in this present work. The field control method relies deeply on accurate acquaintance of the motor signatures. The rotor time constant is particularly hard to measure accurately, and to make matter very inferior because it varies with temperature. The torque control method proficiently controls the IM than field control method. It estimates the stator flux and electric torque by Clarke or αβ transformationand terminal measurements. The mathematical equation of torque control has already been discussed. The approximated stator flux and electric torque are directly controlled by comparing them with their relevant demanded values by using hysteresis comparators. The two comparator’s output will then be used as input signals of an elective switching table. The value of the direct torque control feedback constant k has been computed 6.693.

IV. RESULTS in SYMMETRICAL CONDITION (HEALTHY STANDSTILL CONDITION)

The simulation result in symmetrical condition or healthy standstill condition of the motor is as shown in Fig. 3. Six motor parameters of the induction motor have been used for performance and analysis purpose. These are stator current, rotor current, rotor speed, developed electromagnetic torque, q-axis stator/rotor flux. In the standstill condition of the motor, the load torque and slip set at 15 and 1 respectively. The obtained results show that all the considered motor parameters have been reached in the steady state condition after 0.4 seconds. In the starting of the motor, the first transient peak of the stator current and electromagnetic torque waveforms having highest amplitude when it is decreased and reached in the stable condition. The proposed simulation model is simulated only for 1 sec for clear visualization of transient characteristics. Therefore, the transient characteristic of the motor is clearly visualized.

The squirrel cage induction motor starts and reached in the steady state condition after 0.4 sec and attained Rotating Magnetic Field (RMF) 1430 RPM after 0.4 sec. At the starting, the magnitude of the 50 Hz stator current reaches 96 A peak( 68 A RMS) whereas its steady state value is attained 12.25 A( 8.66 A RMS).

The developed electromagnetic torque waveform reveals that the stable condition is achieved after 0.4 sec. the strong oscillations have been observed in the electromagnetic torque waveform at starting. If it is zoomed, the noisy torque will be observed with a mean value of 15 N.m.

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 3Motor Parameters in Symmetrical Condition(Healthy Standstill Condition), (a) Stator Current, (b) Rotor Current, (c) Rotor Speed, (d) Electromagnetic Torque, (e) q-axis Stator Flux, (f) d-axis Rotor Flux

If we zoomed stator and rotor current waveforms, the harmonics will be displayed multiple of the 1650 Hz switching frequency. This is filtered by stator inductance. Therefore, we can say that the 50 Hz frequency component is dominant. Similarly, q-axis stator and rotor flux is also attained steady state condition after 0.4 sec. After passing transient time, perfectly sinusoidal signal can be observed. Therefore, finally we can say that the proposed simulation model given expected correct results.

Second healthy running condition has also been considered and observed due to the clear revelation of the transient characteristics. In the healthy running condition mode the slip is set at 0.04. At this slip, we can get 1430 RPM rotor speed on full load nominal torque. This is our nominal speed of the

(a)

(b)